The invention relates to a method for increasing the wear resistance of a work piece in accordance with the preamble of claim 1.
In order to increase the wear resistance of a work piece it is known that the loaded surface of the work piece can be protected by means of a material that is of a greater hardness than the work-piece material. Materials that cannot be reshaped, such as hard metal or ceramic materials, called core materials in the following, are particularly suitable for this.
Connections between ceramic materials or hard metals and a metal or non-ferrous metal respectively as the work piece are produced at present by using the basic joining techniques, form-fitting, force-fitting and substance-fitting.
Moreover, connections which cannot be undone are currently mainly realized by means of soldering, welding and shrinkage methods and various bending-reshaping methods, for example flanging or rotatory reshaping under compressive conditions.
It is largely the soldering methods (for example high-temperature or active soldering) and also the welding methods that come into consideration for connections that undergo maximum mechanical stresses.
The disadvantages of the soldering and welding methods are the high costs of production as well as, in most cases, the need to use additional and/or intermediate substances that are matched to the expansion behaviour or the need to carry out structural measures to compensate for the different coefficients of thermal expansion in order to reduce stresses.
The underlying object of the invention is to improve a method for increasing the wear resistance of a work piece in accordance with the preamble of claim 1 in such a way that an extremely durable connection of the core material to the work piece is achieved with simple means and in a less expensive manner. In so doing, the dimensions of the work piece are to be maintained.
In accordance with the invention this object is achieved by connecting the core material to the work piece in a form-fitting manner by means of cold-extrusion or hot-extrusion of the work-piece material.
The method in accordance with the invention is a reshaping method in which a plastic change in the shape of a solid body is effected by means of compression or compression-drawing. The properties of the material and the dimensions of the body are thereby maintained. Cold-extrusion is extrusion without an additional supply of heat to the components or tools before or during the reshaping. However, heat can/will develop as a result of the reshaping. In the case of hot-extrusion, heat is supplied during the extrusion.
The new underlying idea of the method is to use the plastic change in the work-piece material, advantageously steel or non-ferrous metal, during the extrusion, and the non-reshapability of the ceramic sintered materials that have high grain-boundary stability based on dense, high-melting metal oxides, metal carbides and metal nitrides or hard metals and hardened metals, in order to produce a connection which cannot be undone. The sintered materials, the hard metal or the hardened metal of the core materials are shaped in terms of extrusion techniques in such a way that the plastic deformation of the metal/non-ferrous metal is not hindered, but rather is promoted, and the sintered materials or the hard metal are not overloaded with regard to their material properties, specifically the stability properties. Outer and inner contours of the work pieces are then determined by the producibility of the tools.
The connection is clearly less expensive as a result of the use of this new technology (savings in terms of time and materials).
Oxide ceramic materials, such as, for example, aluminium oxide, zirconium oxide, magnesium oxide, mixtures of aluminium oxide and zirconium oxide, silicon nitride, such as, for example, sintered silicon nitride (SSN), hot-pressed (HPSN) or gas pressure-sintered (GPSN) silicon nitride, silicon carbide, such as, for example, densely sintered silicon carbide (SSiC), silicon-infiltrated silicon carbide (SiSiC), dispersion ceramic materials, ceramic silicate materials and also mixtures of titanium carbide and aluminium oxide number among the ceramic sintered materials that are particularly suitable for the present invention. Numbering among these materials within the scope of the present invention are also those materials which contain, in small admixed quantities, magnesium oxide, calcium oxide and yttrium oxide and other sintering aids which are usually added, for example, as grain-growth inhibitors.
In the case of this invention all the hard metals which have mechanical strength values of "sgr"B greater than 350 N/mm2 number among the hard metals which are particularly suitable.
All the metals of the material group 1.2379, for example, number among the hardenable metals which are particularly suitable.
In order to achieve security against torsion or a comparatively high degree of strength of the connection, suitable additional shaped elements such as, for example, rounded-off notches and/or areas or hollow spaces and/or undercuts are worked into the core materials or special surface qualities are produced.
In a particularly advantageous embodiment, the additional shaped elements are constituted by a knurling that is provided on the outside. Advantageously, moreover, the core material tapers towards the outside of the work piece. As a result, even better anchorage of the core material in the work piece is achieved.
In accordance with the invention advantageously an extrusion sleeve liner with a bore, in which a displaceable punch connects the work piece to the core material by means of cold-extrusion or hot-extrusion, is used as the pressing tool. In this connection, the core material is pressed into the work piece, or vice versa the work piece is pressed into the core material, is until the work-piece material is free-flowing under the pressure and flows around the core material. Promoted by the cold-work hardening of the work-piece material that occurs during the reshaping, a permanent, extremely firm connection of the core material with the work piece develops.
Advantageously, a displaceable ejector is provided as an abutment for the work piece or the core material in the bore in the sleeve liner. This ejector is used, after the extrusion, to eject, for example press, the finished work piece out of the sleeve liner.
In a special advantageous embodiment a constriction can also be provided as an abutment for the work piece in the bore in the sleeve liner. It is possible to push the work piece out of the sleeve liner after the extrusion by means of the ejector further described above.
Depending on the required application, it is also advantageous to form the punch as a hollow punch. In this case, the pressure is only applied to an annular outer region of the hollow punch. It is also expedient in specific cases if the punch, at its end that faces the work piece or core material, has a clearance from the bore in the sleeve liner.
In a special embodiment a further displaceable punch, to which force can be applied, is arranged in the punch. By means of this further punch it is possible to control the reshaping of the work-piece material in a purposeful manner.
Advantageously, this method is used in the case of work pieces of valve systems, in particular valve drives of internal combustion engines. Numbering amongst these there is, for example, a tappet that is driven by the cam shaft or else the setting screw of the rocker arm.